Missing eggs under hen????

Ok so we have a duck that went clucky and has been sitting on 10 eggs. They would have been due to hatch 4-5 days ago, since then hens have been stealing the ducks spot and sitting on the eggs. The problem is the last 4-5 days we've gone from 10 eggs down to 4... I candles them and they were all fertile.
Any ideas what could be happening??

Early spring of 2015, I had 4 hens all go broody at the same time. We also had a snake problem in the coop. One of my larger hens was so zoned out while she was on the nest, that a snake crawled under her, swallowed an egg, and went back out the other side without her ever getting up. I just happened to go in the coop while the snake was still in the nest. Half on one side of her and half on the other. We had lost several eggs that week. Never any sign of what happened until I saw that.
We were using milk crates as nests back then. The snake came through a hole in one side, ate the egg, then couldn't get back out through the other side. I had to chop it in to 3 pieces to get it out of the nest.